Electrochemical cell devices are known. These devices are typically made up of a plurality of electrochemical cells, arranged in groups or stacks, and basically serve to either: electrolytically disassociate water or another liquid into its components (i.e., electrolysis cells), or catalytically combine hydrogen or other gaseous fuel and oxygen (i.e., fuel cells), with electricity being either supplied or generated, respectively.
Each electrochemical cell, regardless of its intended use, includes an anode cavity and electrode plate, a cathode cavity and electrode plate, and an electrolyte (which can be any ionically conductive material such as an ion-exchange membrane or liquid contained in a porous matrix) positioned between at least the active area of the electrode plates. The membrane or porous matrix typically has a catalyst layer located on opposing surfaces to facilitate or enhance the electrochemical reaction.
The preferred stacking arrangement or configuration for both electrolysis and fuel cells is hydraulically in parallel, by means of fluid headers and cell manifolds, and electrically in series, by providing a full electrical path along the stacking direction. When these cells are arranged in a group or a stack, fluid cavities are defined by the electrode/electrode plate interface with the membrane on one side and a solid separator sheet at the opposite side. Bipolar assemblies are possible by joining two complementary fluid cavities at the separator sheet interface. General purpose bipolar assemblies typically contain the following sequence of components: electrolyte, electrode, electrode plate, fluid cavity with cell manifolds, separator plate, pressure pad or compensating component, separator plate, fluid cavity with cell manifolds, electrode plate, electrode, and electrolyte from the adjacent cell. Where each component has dimensional tolerances, a compensating component is useful for such multi-component hardware assemblies. As is well known to those skilled in the art, a certain degree of compression is desirable to ensure good electrical conductivity between the different components or parts in contact. This can be achieved by adjusting the overall compression loading during stack assembly. However, when the stack operating conditions are such that it could oppose the contact force, an elastic component such as a pressure pad is typically employed to maintain the minimum contact pressure required during operation. In most instances, the pressure pad also serves to compensate for the dimensional tolerances of the cell components.
Electrochemical cell assemblies or devices, especially those devices utilizing ion-exchange membranes, are best suited to be operated at very high, super-atmospheric pressures. As is well known in the art, when external pressure equalization measures are not employed during the operation of such devices, the resultant pressure differentials both within the cell and between the interior and the exterior of the electrochemical cell device impose considerable strain on the internal cell components in the cell active area and on the peripheral portions of the individual cells. In addition, high demands are placed on the fluid impermeability of the various components of the device and the interfaces therebetween. Where pressure equalization measures typically add to the complexity and cost of the equipment in addition to increasing the overall dimensions and weight of the equipment, the employment of such measures is not always feasible. Accordingly, and in addition to the pressure pad described above, means for accommodating or meeting the demands of such high-pressure operation within the cell itself have been developed for both "overboard" and "cross-cell" high pressure operation.
By way of explanation, since the cell itself contains two fluid cavities, any one or both of them could be operated at above-ambient pressure. The "overboard" pressure capability refers to a cell assembly with both fluid cavities being at essentially the same pressure above ambient. In this operating mode, there is relatively little stress imposed on the electrolyte cell component and it is the only way a cell using liquid electrolyte contained in a porous matrix could be used. Conversely, the "cross-cell" pressure capability refers to a cell assembly where one fluid cavity is at a substantially higher pressure than the other. In this operating mode, stress is exerted across the electrolyte cell component in direct proportion to the pressure difference between the two fluid cavities. "Cross-cell" pressure capability of porous matrix electrolyte structures is typically in the range of 1 kilopascal (kPa) while, properly supported polymer membrane electrolyte structure is currently capable of more than 21 megapascals (MPa) across the cell.
In particular regard to high-pressure electrochemical cell devices employing ion-exchange membranes, the anode and cathode electrode plates of such devices are usually made up of at least two plate-shaped components and are used in conjunction with finer mesh screens and/or porous sheets, that lend additional support to the membrane while allowing easy material access to and from the electrode. These high-pressure cell components are basically constructed to include a solid, fluid impervious frame of a suitable shape and size and a fluid pervious central portion bounded by the frame. The frame of each component includes a plurality of through apertures or slots. Upon assembly of a unit cell, such apertures or slots are aligned so as to collectively constitute fluid supply and discharge flow paths or manifolds. In addition, the frame of the plate-shaped components of the electrode plates contain channels that serve to connect the through apertures or slots and the fluid pervious central portion.
U.S. Pat. No. 5,316,644 to Titterington et al., which is incorporated herein by reference, discloses an improved electrochemical cell electrode plate preferably made up of at least two plate-shaped components. As described above, each component has a fluid pervious central portion and a solid frame, circumferentially surrounding the central portion, and provided with at least one through aperture. The frame is also provided with a plurality of rows of separate slots, with the slots of one of the two components partially overlapping those of the other component so as to form a passage connecting the through apertures or slots and the central portions that dictates a tortuous path. (See, Col. 4, lines 21 to 26.)
The fluid pervious central portion or active area of each plate-shaped component of the improved electrode plate of Titterington et al. constitute a mesh or network of diamond-shaped openings. The plate-shaped components are stacked with the long dimensions of the diamonds at 90.degree. angles relative to each other. However, it has been observed that large, through openings, formed as a result of this stacking technique, leave adjacent cell components essentially unsupported in areas.
Porous sheets are employed as fluid permeable mechanical supports in high-pressure electrochemical cell devices to prevent the ion-exchange membrane from extruding, under the stress caused by the "cross-cell" pressure, into an adjacent screen located on the side of the membrane having the lower pressure, which would cause electrolyzer failure. U.S. Pat. No. 5,296,109 to Carlson et al., which is also incorporated herein by reference, discloses an improved water electrolyzer that employs such a porous sheet. In particular, Carlson et al. disclose the use of a porous metal membrane support, having multiple pore sizes, with a water electrolyzer. The porous membrane supports can be interposed between the anode electrode plate, the cathode electrode plate, or both, and the fine mesh screens. These membrane supports are typically prepared by powder metal sintering.
The operating pressure, whether "overboard" or "cross-cell", opposes the initial compression load imposed on a stack assembly. As is well known, good electrical conductivity requires that a minimum contact pressure is maintained at all times. As set forth hereinabove, an elastic component, commonly referred to as a compression or pressure pad, is included in an electrochemical cell assembly in order to effect uniform contact pressure over the entire active area of the anode and cathode electrode plates and to compensate for variations in dimensional tolerances. Related patents U.S. Pat. No. 5,366,823 and U.S. Pat. No. 5,466,354, both to Leonida et al., which are incorporated herein by reference, disclose an improved compression pad described as a high-pressure compatible, electrically conductive porous metal pad useful in maintaining uniform contact at cell operating pressures above 10.34 megapascals (MPa). The porous metal pad (e.g., nickel felt pressure pad) has sufficient porosity to attain an elastic strain of from about 3% to about 40% at a pressure of about 17.24 MPa.
The most commonly used commercially available ion-exchange membranes are NAFION.RTM. perfluorocarbon sulfonic acid membranes, produced by E.I. DuPont de Nemours and Co., Inc. of Wilmington, Del. These membranes are used for high pressure electrochemical cells because of their chemical stability. However, NAFION.RTM. membranes are an ionically conductive derivative of TEFLON.RTM. perfluoroalkoxy (PFA) resins and, like the majority of materials in this category, will creep or flow under a constant load.
It has been found that, even when the above-described electrode plates, finer mesh screens, porous sheets, and compression pads are employed in high-pressure electrochemical cell devices, membrane creep still occurs at high loads, which can result in high-pressure fluid leaks and/or loss of electrical isolation across the cell(s).
In fabricating high-pressure electrochemical cell devices, bonding materials are typically thinly applied to the solid frame or sealing surfaces of the cell components that have been pre-treated to enhance adhesion. The components are then assembled and the resulting assembly laminated by subjecting it to elevated temperatures and pressures.
It has been observed, however, that the thinly applied bonding materials of such laminated assemblies form grainy coatings with many microscopic voids.
Accordingly, it is a general object of the present invention to avoid the above-referenced disadvantages of the prior art.
More particularly, it is an object of the present invention to provide a high-pressure electrochemical cell device that is capable of successfully operating at extremely high pressures for long periods of time.
It is a more particular object to provide a high-pressure electrochemical cell device that demonstrates reduced membrane creep or flow when subjected to high axial loadings and that demonstrates improved adhesion between cell components.
It is an even more particular object to provide an electrically non-conductive plate structure or grip sheet that effects such reduced membrane creep or flow.